$00/0: 6510 On-chip Data Direction Register
+----------+---------------------------------------------------+
| Bit 7 | Undefined |
| Bit 6 | Only available on C128, otherwise undefined |
| Bits 0-5 | 1 = Output, 0 = Input (see $01 for description) |
+----------+---------------------------------------------------+
Default Value is $2F/47 (%00101111).
Kernal-Reference:
LDA $00,X : $F5E2
STA $00 : $FDDB
$00,X : $B4D7
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